This invention relates to a transmission power control method and apparatus. More particularly, the invention relates to a transmission power control method and apparatus in a W-CDMA communication system, etc., for comparing the error rate of receive data and a target error rate on the receiving side, controlling a target SIR which is a target ratio of signal wave to interference wave and causing the transmitting side to control transmission power in such a manner that measured SIR will agree with the target SIR.
In order to distinguish a channel by a spreading code in W-CDMA mobile communications, multiple channels can share a single frequency band. In an actual mobile communications environment, however, a receive signal is susceptible to interference from its own channel and from other channels owing to delayed waves ascribable to multipath fading and radio waves from other cells, and this interference has an adverse influence upon channel separation. Further, the amount of interference sustained by a receive signal varies with time owing to momentary fluctuations in reception power ascribable to multipath fading and changes in the number of users communicating simultaneously. In an environment in which a receive signal is susceptible to noise that varies with time in this fashion, it is difficult for the quality of a receive signal in a mobile station linked to a base station to be maintained at a desired quality in a stable manner.
In order to follow up a change in number of interfering users and a momentary fluctuation caused by multipath fading, inner-loop transmission power control is carried out. In such control, the signal-to-interference ratio (SIR) is measured on the receiving side and the measured value is compared with a target SIR, whereby control is exercised in such a manner that the SIR on the receiving side will approach the target SIR.
Inner-Loop Transmission Power Control
FIG. 19 is a diagram useful in describing inner-loop transmission power control. Here only one channel is illustrated. A spread-spectrum modulator 1a of a base station 1 spread-spectrum modulates transmit data using a spreading code conforming to a specified channel. The spread-spectrum modulated signal is subjected to processing such as orthogonal modulation and frequency conversion and the resultant signal is input to a power amplifier 1b, which amplifies this signal and transmits the amplified signal toward a mobile station 2 from an antenna. A despreading unit 2a in the receiver of the mobile station applies despread processing to the receive signal and a demodulator 2b demodulates the receive data. A SIR measurement unit 2c measures the power ratio between the receive signal and an interference signal and a comparator 2d compares target SIR and measured SIR. If the measured SIR is greater than the target SIR, the comparator creates a command that lowers the transmission power by TPC (Transmission Power Control) bits. If the measured SIR is less than the target SIR, the comparator creates a command that raises the transmission power by the TPC bits. The target SIR is a SIR value necessary to obtain, e.g., 10−3 (error occurrence at a rate of once every 1000 times). This value is input to the comparator 2d from a target-SIR setting unit 2e. A spread-spectrum modulator 2f spread-spectrum modulates the transmit data and TPC bits. After spread-spectrum modulation, the mobile station 2 subjects the signal to processing such as a DA conversion, orthogonal modulation, frequency conversion and power amplification and transmits the resultant signal toward the base station 1 from an antenna. A despreading unit 1c on the side of the base station applies despread processing to the signal received from the mobile station 2, and a demodulator 1d demodulates the receive data and TPC bits and controls the transmission power of the power amplifier 1b in accordance with a command specified by the TPC bits.
FIG. 20 is a diagram showing the structure of a DPCH (Dedicated Physical Channel) frame of an uplink standardized by the 3rd Generation Partnership Project (referred to as “3GPP” below). There are a DPDCH channel (Dedicated Physical Data Channel) on which only transmit data is transmitted, and a DPCCH channel (Dedicated Physical Control Channel) on which a pilot and control data such as the TPC bit information described in FIG. 19 are multiplexed and transmitted. One frame of the uplink has a duration of 10 ms and is composed of 15 slots (slot #0 to slot #14). The DPDCH channel is mapped to an orthogonal I channel of QPSK modulation, and the DPCCH channel is mapped to an orthogonal Q channel of QPSK modulation. Each slot of the DPDCH channel consists of n bits, and n varies in accordance with the symbol rate. Each slot of the DPCCH control channel that transmits the control data consists of ten bits, has a fixed symbol rate of 15 ksps and transmits a pilot PILOT, transmission power control data TPC, a transport format combination indicator TFCI and feedback information FBI.
Outer-Loop Transmission Power Control
Owing to changes in traveling velocity during communication and changes in the propagation environment ascribable to travel, the SIR that is necessary to obtain a desired quality (the block error rate, or BLER) is not constant. It should be noted that the BLER is the ratio between the total number of transport blocks (TrBk) and number of TrBks for which a CRC error has occurred over a fixed period of time.
In order to deal with these changes, the BLER is observed and control is exercised so as to increase the target SIR if the observed value of BLER is inferior to the target BLER and decrease the target SIR if the observed value of BLER is superior to the target BLER. Control that thus changes the target SIR adaptively in order to achieve the desired quality is well known as outer-loop transmission power control (outer-loop TPC).
FIG. 21 is a block diagram of well-known outer-loop control. This scheme is such that a signal that has been transmitted from a base station 3 is demodulated by a demodulator 4a and then subsequently decoded by an error-correction decoder 4b. Thenceforth, in a CRC detector 4c, the signal is split into transport blocks TrBk, after which CRC error detection is carried out on a per-TrBk basis. The result of error detection of each transport block TrBk is sent to a target-SIR controller 4d. 
In W-CDMA as presently standardized, encoding is performed on the transmitting side in the manner illustrated in FIG. 22. Specifically, if a plurality (N) of transport blocks TrBk exist in a unit transmission time (Transmission Time Interval, or TTI), a CRC add-on circuit on the transmitting side generates a CRC (Cyclic Redundancy Code) error detection code for every transport block TrBk and adds this onto the transmit data. An encoder joins the N-number of transport blocks TrBk having the attached CRCs and encodes the blocks by error correcting coding such as convolutional coding or turbo coding. On the receiving side the error correcting decoder 4b subjects the receive data to error-correction decoding processing and inputs the result of decoding to the CRC detector 4c, and the CRC detector 4c performs CRC error detection for every transport block TrBk constituting the result of decoding and inputs the results of error detection to the target-SIR controller 4d. 
Immediately after a dedicated channel DCH (Dedicated CH) call is placed to the target-SIR controller 4d, a host application specifies the required BLER of each service depending upon the service type of the DCH, such as voice, packet or unrestricted digital. Let BLERquality represent the required BLER, let Tmax represent the number of transport blocks TrBk for which BLER is measured, let Sinc (dB) represent an update quantity for raising the target SIR in a case where the measured BLER is inferior to the required BLER, and let Sdec (dB) represent an update quantity for lowering the target SIR in a case where the measured BLER is superior to the required BLER. If there is even one CRC NG (CRC error) in Tmax-number of BLER measurement periods, the target SIR is updated by Sinc. If CRC OK holds throughout, the target SIR is updated by Sdec. When this is observed in total, the target SIR settles stabilizes at a fixed level. This is the fundamental concept of outer-loop control. According to this concept, the values Sinc, Sdec and Tmax are decided so as to satisfy the following equation:(1−BLERquality)Tmax×Sdec=[1−(1−BLERquality)Tax]×Sinc It should be noted that (1−BLERquality)Tmax indicates the probability that the CRC check will be correct Tmax-times in succession, and [1−(1−BLERquality)Tmax] indicates the probability that there will be even one CRC check error in Tmax times.
More specifically, BLER measurement is performed with regard to Tmax-number of TrBks. If CRC OK is obtained for all TrBks, the target SIR is updated by Sdec. If there is even one CRC NG (CRC error), then the target SIR is updated by Sinc.
The values of Sinc, Sdec and Tmax are values uniquely decided by the required BLER of each service. Accordingly, in a case where a plurality of transport channels (abbreviated to “TrCH” below) have been mapped to a single physical channel (abbreviated to “PhCH” below), then values of Sinc, Sdec and Tmax will exist for each TrCH.
Standards in 3GPP
With regard to downlink transmission from a base station in 3GPP TS25.101, it is stipulated that control for lowering transmission power from the base station be carried out in a case where a transmission has been made at power above the required quality BLERquality of BLER and that control for raising the transmission power from the base station be carried out in a case where a transmission has been made at power below the required quality BLERquality of BLER. A time T1 over which quality is pulled in to the required quality of BLER at such time has been stipulated. That is, T1 interval=500 ms has been stipulated.
{circle around (1)} Standard 1
It is necessary to pull DPCH SIR into the range −3 dB to +4 dB in order to satisfy the required BLER within the T1 time period after downlink power control is started (this is referred to as “initial pull-in”).
{circle around (2)} Standard 2
There is also a T2 interval=500 ms stipulation. It is also necessary to pull the DPCH SIR value, which has been decided by initial pull-in, into the range −3 dB to +1 dB within 500 ms following the T1 interval (this is referred to as the “steady state”).
With the prior art technique, the period at which the target SIR is updated is uniquely decided after Tmax-number of BLER measurements. Consequently, if this measurement period does not fall within the T1 time period, the target SIR will not be updated from the initial target SIR in a period greater than the T1 time period, and hence there is the possibility that initial pull-in cannot be achieved as specified in Standard 1.
Conversely, in a case where the period at which the target SIR is updated is too short, the frequency at which the target SIR is updated will be high. As a result, there is the possibility that pull-in in the steady state will depart from and overshoot or undershoot the stipulated range.
{circle around (3)} Standard 3
Further, 3GPP TS25.101 recites a standard to the effect that measured BLER fall within a range of ±30% of the required BLER.
{circle around (1)} Problem 1
With the prior art technique, the updating period of the target SIR and the updated value of the target SIR are decided uniquely for every required BLER specified on a pre-service basis. During the connection of a certain service, therefore, these values are fixed.
In actuality, however, the propagation environment changes from moment to moment owing to the environment in which the mobile station is placed. Consequently, in an environment in which the BLER deteriorates greatly, as when the mobile station travels in a very poor propagation environment, there is a possibility that the measured BLER will not be able to follow up Standard 3 concerning the required BLER.
{circle around (2)} Problem 2
Furthermore, when the entire apparatus is considered, there are instances where the antenna characteristic, the characteristic of the receiver that down-converts a high-frequency signal to a baseband signal, the characteristic of the demodulator that applies despread processing to the baseband signal, and the characteristics of the various components of the decoder that applies an error correction to the symbol signal that has been despread, differ depending upon the specifications of the apparatus. Consequently, the characteristics of a certain apparatus will differ from those of an apparatus having other specifications. In other words, the BLER vs. SCR characteristic will differ depending upon the apparatus specifications, etc. For example, reception sensitivity varies depending upon whether the apparatus has whip antenna in terms of the antenna configuration, or the decoding characteristic varies owing to a disparity in soft-decision bit width in the error correcting unit, and therefore the BLER vs. SCR characteristic differs depending upon whether the apparatus has a whip antenna or because of the disparity in soft-decision bit width. Therefore, with the conventional method of controlling target SIR based upon the prescribed BLER vs. SCR characteristic, a situation arises in which the required BLER cannot be satisfied depending upon the apparatus.
{circle around (3)} Problem 3
In a case where a plurality of TrCHs are mapped onto a single physical channel, e.g., a case where two TrCHs TrCH1, TrCH2 are multiplexed, the SIR with respect to the required BLER (namely the BLER vs. SIR characteristic) of each TrCH differs, as illustrated at (A) and (B) of FIG. 23. That is, the SIR that satisfies the required BLER of TrCH1 is A and the SIR that satisfies the required BLER of TrCH2 is B. In a case where the TrCHs are multiplexed as is, therefore, the required qualities of both TrCHs cannot be satisfied simultaneously.
There is a 3GPP standard for solving this problem. This 3GPP standard applies weighting to a rate-matching attribute parameter (RM) to satisfy the required quality of each TrCH and makes the characteristic of a TrCH having good quality and the characteristic of a TrCH having poor quality approach each other. A method of weighting a rate-matching attribute parameter will be described with reference to (A) of FIG. 24, in which the two characteristics of (A) and (B) of FIG. 23 have been superimposed. Let the SIR that satisfies the required BLER of TrCH1 be A (dB), let the SIR that satisfies the required BLER of TrCH2 be B (dB), and let the SIR that satisfies the required BLER when both TrCHs are multiplexed be a (dB). The difference between SIR A (dB) vs. the required BLER of TrCH1 and SIR a (dB) that satisfies the required BLER when the two TrCHs are multiplexed is α−A (dB). Similarly, the difference between SIR B (dB) vs. the required BLER of TrCH2 and SIR α (dB) that satisfies the required BLER when the two TrCHs are multiplexed is B−α(dB).
Values that depart from the dB calculation of these differential values correspond to the ratio between new rate-matching attributes RM1′, RM2′ and rate-matching attributes RM1, RM2 prevailing prior to weighting. Accordingly, the following equations hold:RM1′/RM1=10(α−A)/10 RM2′/RM2=10(B−α)/10 By deciding the new rate-matching attributes RM1′, RM2′ of TrCH1, TrCH2 so as to solve the above equations and applying weighting by these rate-matching attributes, it is possible to satisfy the required BLER for when a plurality of TrCHs having different required BLERs are multiplexed. More specifically, in a case where a plurality of TrCHs have been mapped to a single physical channel, the SIR vs. BLER characteristic of each TrCH changes as indicated at (B) of FIG. 24 owing to weighting by the above-mentioned rate-matching attributes. If we let BLERx represent the required BLER of each TrCH1, TrCH2 at the time of multiplexing, the required BLER (=BLERx) of each TrCH can be satisfied by adopting SIR (=α) vs. BLERx as the target SIR based upon (B) of FIG. 24.
However, the prior art in which the parameter α decided uniquely for the service of each TrCH is adopted as the target SIR with respect to the required BLER is such that in a case where a data transmission is performed on a low-quality TrCH alone, the required BLER of this TrCH can no longer be satisfied. That is, if data is transmitted solely by the low-quality TrCH2, α<B will hold and the required BLER of the low-quality TrCH can no longer be satisfied. It should be noted that since α>B holds, the required BLER can be satisfied with regard to TrCH1.
{circle around (4)} Problem 4
Furthermore, in a case where a plurality of TrCHs are multiplexed, there are instances where the standard (Standard 3) to the effect that the measured BLER fall within the range of ±30% of the required BLER is no longer satisfied. For example, in a case where the required BLER of the TrCH of a certain service is 5×10−3, the standard range of this TrCH is 3.5×10−3 to 6.5×10−3, and in a case where the required BLER of the TrCH of another service is 5×10−2, the standard range of this TrCH is 3.5×10−2 to 6.5×10−2. The ranges of the two standards do not overlap. In such case, Standard 3 can no longer be satisfied.
There is prior art concerning transmission power control in a mobile communication system (Patent Reference 1: Japanese Patent Application Laid-Open No. 2002-16545). According to this prior art, control entails detecting receive error rate of a receive signal; comparing the receive error rate and a target receive error rate set in advance; correcting, based upon the result of the comparison, a target reception signal power vs. interference power ratio (SIR) to serve as a target or a target reception power value to serve as a target; and controlling transmission power on the transmitting side based upon the corrected SIR or target reception power value.
Further, there is other prior art concerning transmission control (Patent Reference 2: WO 97/50197). This prior art measures the error rate of a receive signal and changes the target SIR based upon the error rate. The error rate of the receive signal is acquired by detecting a frame unit using a CRC signal or by detecting error of a known pilot signal that has been inserted at a fixed period.
However, these examples of the prior art cannot control the target SIR so as to satisfy the required BLER of apparatuses having different characteristics. Further, these examples of the prior art cannot update the target SIR and control the updating period in accordance with the propagation environment even if the propagation environment changes. The end result is that the required BLER cannot be satisfied when the propagation environment fluctuates. Furthermore, these examples of the prior art are such that even in a case where a sole TrCH is used and in a case where a plurality of TrCHs are multiplexed, the target SIR cannot be updated in such a manner that the required BLER can be satisfied. In addition, the target SIR also cannot be controlled so as to satisfy the standards.